The present invention relates to a metal-organic chemical vapor deposition (referred to hereinafter as "MOCVD") process for forming a layer of a compound semiconductor comprising elements belonging to Group II and Group VI of periodic table (referred to simply hereinafter as a "Group II-VI compound semiconductor"), and to a process for fabricating light-emitting devices such as semiconductor lasers and light-emitting diodes (LEDs) by means of MOCVD process.
The Group II-VI compound semiconductors, particularly ZnMgSSe compound semiconductors that are believed to be the key material, are now actively studied as promising crystalline materials for the advanced short-wave semiconductor lasers. Because ZnMgSSe compound semiconductors have a large energy band gap and also a smaller index of refraction as compared with that of ZnSSe, they are suitable for use in the clad layers of blue-emitting semiconductor lasers.
A layer of a Group II-VI compound semiconductor is fabricated by depositing the Group II-VI compound semiconductor layer on a gallium arsenide (GaAs) substrate by means of molecular beam epitaxy (MBE). An MBE process comprises heating each of the starting materials charged into molecular beam sources (e.g, Knudsen cells) to generate molecular beams, and irradiating the thus obtained molecular beams to the substrate to form a layer of the compound semiconductor.
However, in case of vaporizing sulfur, i.e., one of the materials for forming a ZnMgSSe compound semiconductor, by means of MBE, the temperature control of the molecular beam source tend to be unstable because the beam sources containing therein sulfur or selenium are heated at such a low temperature of about 200.degree. C. or lower. This makes it difficult to control the composition of the compound semiconductor layer. Particularly in case of fabricating a semiconductor laser, the fluctuation in the composition of the compound semiconductor layer not only impairs the light-emitting characteristics of the semiconductor layer, but also makes it impossible to obtain a laser which emits light as designed.
A compound semiconductor layer can be otherwise fabricated by MOCVD process. In MOCVD process, the composition of the compound semiconductor layer is controlled by varying the flow rate of the gaseous starting materials. Thus, the composition of the compound semiconductor layer can be controlled with higher accuracy. Moreover, a compound layer with a grated structure, which is almost impossible to form by an MBE process, can be easily fabricated by controlling the flow rate of the gaseous material constituting the compound semiconductor layer.
In an MOCVD process, bis(cyclopentadienyl)magnesium [Mg(C.sub.5 H.sub.5).sub.2, referred to simply as "(Cp).sub.2 Mg"] has been used conventionally as the organometallic material for supplying magnesium. Referring to FIG. 8, it can be seen that the vapor pressure of (Cp).sub.2 Mg is low. More specifically, the vapor pressure of (Cp).sub.2 Mg in the temperature range of 370.degree. K. or lower for an ordinary thermostatic cell is found to be 1.times.10.sup.-3 Torr (0.133 Pa) or lower. Thus, although (Cp).sub.2 Mg can be used favorably as a dopant, it is not suitable for use as a gaseous material for growing the base crystal. In contrast to (Cp).sub.2 Mg, dimethylzinc (DMZn) and dimethylselenium (DMSe) can be readily used as the gaseous material for depositing the base crystal because they yield a vapor pressure in the range of from several tens to several hundreds of Torr.
Thus, as mentioned in the foregoing, although there is a known process for fabricating a ZnSe compound semiconductor layer by an MOCVD process, a technique for growing crystals of a ZnMgSSe compound semiconductor on a substrate by means of MOCVD is yet to be developed.